ASK vs. FSK vs.
PSK-Difference between ASK, FSK, PSK modulation
This
page on ASK vs. FSK vs. PSK provides difference between ASK, FSK, PSK
modulation types. All these are digital modulation techniques. Unlike Analog
modulation, here input is in digital binary form. The other input is the RF
carrier. Input binary data is referred as modulating signal and output is
referred as modulated signal.
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ASK
The
short form of Amplitude Shift Keying is referred as ASK. It is the digital
modulation technique. In this technique, amplitude of the RF carrier is varied
in accordance with baseband digital input signal. The figure depicts operation
of ASK modulation. As shown in the figure, binary 1 will be represented by
carrier signal with some amplitude while binary 0 will be represented by
carrier of zero amplitude(i.e. no carrier).
Fig.1
ASK Modulation
ASK
modulation can be represented by following equation:
s(t) =
A2* cos(2*π*fc*t) for Binary Logic-1
s(t) =
A1* cos(2*π*fc*t) for Binary Logic-0
Here
A2>A1
Signaling
used is ON-OFF signaling.
Bandwidth
requirement for ASK is:
BW = 2/Tb
= 2*Rb
Often in
ASK modulation, binary-1 is represented by carrier with amplitude-A2 and
binary-0 is represented by carrier with amplitude-A1. Here A2 is greater in
magnitude compare to A1. The form of ASK where in no carrier is transmitted
during the transmission of logic zero is known as OOK modulation (On Off Keying
modulation). This is shown in the figure-1. Refer OOK vs ASK modulation
>> which compares OOK vs. ASK and depicts difference between OOK and ASK
modulation types with signal diagrams.
• In ASK
probability of error (Pe) is high and SNR is less.
• It has
lowest noise immunity against noise.
• ASK is
a bandwidth efficient system but it has lower power efficiency.
FSK
The
short form of Frequency Shift Keying is referred as FSK. It is also digital
modulation technique. In this technique, frequency of the RF carrier is varied
in accordance with baseband digital input. The figure depicts the FSK
modulation. As shown, binary 1 and 0 is represented by two different carrier
frequencies. Figure depicts that binary 1 is represented by high frequency 'f1'
and binary 0 is represented by low frequency 'f2'.
Fig.2
FSK
Binary
FSK can be represented by following equation:
s(t) =
A* cos(2*π*f1*t) for Binary 1
s(t) =
A* cos(2*π*f2*t) for Binary 0
In FSK modulation,
NRZ signaling method is used. Bandwidth requirement in case of FSK is:
BW =
2*Rb + (f1-f2)
• In
case of FSK, Pe is less and SNR is high.
• This
technique is widely employed in modem design and development.
• It has
increased immunity to noise but requires larger bandwidth compare to other
modulation types.
In order
to overcome drawbacks of BFSK (Two level Binary FSK) , multiple FSK modulation
techniques with more than two frequencies have been developed. In MFSK
(Multiple FSK), more than one bits are represented by each signal elements.
Refer
2FSK and 4FSK Modulation types.
PSK
The
short form of Phase Shift Keying is referred as PSK. It is digital modulation
technique where in phase of the RF carrier is changed based on digital input.
Figure depicts Binary Phase Shift Keying modulation type of PSK. As shown in
the figure, Binary 1 is represented by 180 degree phase of the carrier and
binary 0 is represented by 0 degree phase of the RF carrier.
Fig.3
PSK
Binary
PSK can be represented by following equation :
If s(t)
= A*cos(2*π*fc*t) for Binary 1 than
s(t) =
A*cos(2*π*fc*t + π) for Binary 0
In PSK
modulation, NRZ signaling is used. Bandwidth requirement for PSK is:
BW = 2 *
Rb = 2 * Bit rate
• In
case of PSK probability of error is less. SNR is high.
• It is
a power efficient system but it has lower bandwidth efficiency.
• PSK
modulation is widely used in wireless transmission.
• The
variants of basic PSK and ASK modulations are QAM, 16-QAM, 64-QAM and so on.
Advantages and
Disadvantages of ASK, FSK and PSK
ASK Advantages
| ASK Disadvantages | Amplitude Shift Keying
This
page covers advantages and disadvantages of ASK.ASK stands for Amplitude Shift
Keying. Both ASK advantages and ASK disadvantages are covered.
Following
are the silent features of ASK modulation.
• ASK is
digital modulation technique in which carrier is analog and data to be
modulated is digital. Modulated output is analog.
• Here
strength or amplitude of carrier signal is varied to represent binary 1 and
binary 0 data inputs; While frequency and phase of the carrier signal remain
constant. Voltage levels are left to designers of the modulation system.
Figure-1:
ASK Modulation
ASK Advantages
Following
points summarizes ASK advantages:
➨It offers high bandwidth efficiency.
➨It has simple receiver design.
➨ASK modulation can be used to transmit digital data
over optical fiber.
➨ASK modulation and ASK demodulation processes are
comparatively inexpensive.
➨Its variant OOK is used at radio frequencies to
transmit more codes.
ASK
Disadvantages
Following
points summarizes ASK disadvantages:
➨It offers lower power efficiency.
➨ASK modulation is very susceptible to noise interference.
This is due to the fact that noise affects the amplitude. Hence another
alternative modulation technique such as BPSK which is less susceptible to
error than ASK is used.
FSK
Advantages | FSK Disadvantages | Frequency Shift Keying
This
page covers advantages and disadvantages of FSK. It mentions FSK advantages or
benefits and FSK disadvantages or drawbacks. FSK stands for Frequency Shift
Keying.
What is FSK?
Introduction:
It is a
digital modulation technique which shifts the frequency of the carrier with
respect to binary data signal. FSK stands for Frequency Shift Keying. The FSK
modulation technique uses two different carrier frequencies to represent binary
1 and binary 0.
As shown
in the figure-1, carrier frequency f1 represents binary data one and carrier
frequency f2 represents binary data zero. Here amplitude and phase of the
carrier remain constant while carrier frequency is changed. Binary FSK (BFSK)
can be represented by following mathematical equation:
s(t) =
A* cos(2*π*f1*t) for Binary 1
s(t) =
A* cos(2*π*f2*t) for Binary 0
In this
equation, f2 and f2 are offset from carrier frequency (Fc) by equal but
opposite amounts.
Following
are the typical applications of FSK modulation.
• It is
used on voice grade lines for data rates up to 1200 bps.
• It is
used for high frequency radio transmission from 3 to 30 MHz.
• It is
also used in coaxial cable based LAN (Local Area Network) at higher
frequencies.
Benefits or
advantages of FSK
Following
are the benefits or advantages of FSK:
➨It has lower probability of error (Pe).
➨It provides high SNR (Signal to Noise Ratio).
➨It has higher immunity to noise due to constant
envelope. Hence it is robust against variation in attenuation through channel.
➨FSK transmitter and FSK receiver implementations are
simple for low data rate application.
Drawbacks
or disadvantages of FSK
Following are
the disadvantages of FSK:
➨It uses larger bandwidth compare to other modulation
techniques such as ASK and PSK. Hence it is not bandwidth efficient.
➨The BER (Bit Error Rate) performance in AWGN channel is
worse compare to PSK modulation.
In order
to overcome drawbacks of BFSK, multiple FSK modulation techniques with more
than two frequencies have been developed. In MFSK (Multiple FSK), more than one
bits are represented by each signal elements.
PSK Advantages
| PSK Disadvantages | Phase Shift Keying
This
page covers advantages and disadvantages of PSK. It mentions PSK advantages or
benefits and PSK disadvantages or drawbacks. PSK stands for Phase Shift Keying.
What is PSK?
Introduction:
It is a
digital modulation technique which uses phase of the analog carrier to
represent digital binary data. Phase of the carrier wave is changed according
to the binary inputs (1 or 0). In two level PSK, difference of 180 phase shift
is used between binary 1 and binary 0.
There
are many different types of modulation techniques which utilizes this concept
to transmit digital binary data. It include two level PSK (i.e. BPSK), Four
level PSK (i.e. QPSK) etc. Some techniques employ both amplitude and phase
variation to represent binary data such as 16-QAM, 64-QAM, 256-QAM etc. Two
level PSK represents single bit by each signaling elements while four level PSK
represents two bits by each signaling elements and so on. 8-PSK represents
three bits by each signaling elements.
Following
are the equations used to represent BPSK.
➨s(t) = A*cos(2*π*fc*t) for Binary 1 than
➨s(t) = A*cos(2*π*fc*t + π) for Binary 0
As
mentioned there are many variants of PSK modulation. Each of these PSK types
have different advantages and disadvantages. We will have a look at common
advantages and disadvantages of PSK techniques.
Benefits or
advantages of PSK
Following
are the benefits or advantages of PSK:
➨It carries data over RF signal more efficiently compare
to other modulation types. Hence it is more power efficient modulation
technique compare to ASK and FSK.
➨It is less susceptible to errors compare to ASK
modulation and occupies same bandwidth as ASK.
➨Higher data rate of transmission can be achieved using
high level of PSK modulations such as QPSK (represents 2 bits per
constellation), 16-QAM (represents 4 bits per constellation) etc.
Drawbacks or
disadvantages of PSK
Following
are the disadvantages of PSK:
➨It has lower bandwidth efficiency.
➨The binary data is decoded by estimation of phase
states of the signal. These detection and recovery algorithms are very complex.
➨Multi-level PSK modulation schemes (QPSK, 16QAM etc.)
are more sensitive to phase variations.
➨It is also one form of FSK and hence it also offers
lower bandwidth efficiency compare to ASK modulation type.
Digital Phase
Modulation: BPSK, QPSK, DQPSK
Digital
phase modulation is a versatile and widely used method of wirelessly
transferring digital
data.
In the
previous page, we saw that we can use discrete variations in a carrier’s amplitude
or frequency as a way of representing ones and zeros. It should come as no
surprise that we can also represent digital data using phase; this technique is
called phase shift keying (PSK).
Binary Phase
Shift Keying
The most
straightforward type of PSK is called binary phase shift keying (BPSK), where
“binary” refers to the use of two phase offsets (one for logic high, one for
logic low).
We can
intuitively recognize that the system will be more robust if there is greater
separation between these two phases—of course it would be difficult for a
receiver to distinguish between a symbol with a phase offset of 90° and a
symbol with a phase offset of 91°. We only have 360° of phase to work with, so
the maximum difference between the logic-high and logic-low phases is 180°. But
we know that shifting a sinusoid by 180° is the same as inverting it; thus, we
can think of BPSK as simply inverting the carrier in response to one logic
state and leaving it alone in response to the other logic state.
To take
this a step further, we know that multiplying a sinusoid by negative one is the
same as inverting it. This leads to the possibility of implementing BPSK using
the following basic hardware configuration:
However,
this scheme could easily result in high-slope transitions in the carrier
waveform: if the transition between logic states occurs when the carrier is at
its maximum value, the carrier voltage has to rapidly move to the minimum
voltage.
High-slope
events such as these are undesirable because they generate higher-frequency
energy that could interfere with other RF signals. Also, amplifiers have
limited ability to produce high-slope changes in output voltage.
If we
refine the above implementation with two additional features, we can ensure
smooth transitions between symbols. First, we need to ensure that the digital
bit period is equal to one or more complete carrier cycles. Second, we need to
synchronize the digital transitions with the carrier waveform. With these
improvements, we could design the system such that the 180° phase change occurs
when the carrier signal is at (or very near) the zero-crossing.
QPSK
BPSK
transfers one bit per symbol, which is what we’re accustomed to so far.
Everything we’ve discussed with regard to digital modulation has assumed that
the carrier signal is modified according to whether a digital voltage is logic
low or logic high, and the receiver constructs digital data by interpreting
each symbol as either a 0 or a 1.
Before
we discuss quadrature phase shift keying (QPSK), we need to introduce the
following important concept: There is no reason why one symbol can transfer
only one bit. It’s true that the world of digital electronics is built around
circuitry in which the voltage is at one extreme or the other, such that the
voltage always represents one digital bit. But RF is not digital; rather, we’re
using analog waveforms to transfer digital data, and it is perfectly acceptable
to design a system in which the analog waveforms are encoded and interpreted in
a way that allows one symbol to represent two (or more) bits.
QPSK is
a modulation scheme that allows one symbol to transfer two bits of data. There
are four possible two-bit numbers (00, 01, 10, 11), and consequently we need
four phase offsets. Again, we want maximum separation between the phase
options, which in this case is 90°.
The
advantage is higher data rate: if we maintain the same symbol period, we can
double the rate at which data is moved from transmitter to receiver. The
downside is system complexity. (You might think that QPSK is also significantly
more susceptible to bit errors than BPSK, since there is less separation
between the possible phase values. This is a reasonable assumption, but if you
go through the math it turns out that the error probabilities are actually very
similar.)
Variants
QPSK is,
overall, an effective modulation scheme. But it can be improved.
Phase
Jumps
Standard
QPSK guarantees that high-slope symbol-to-symbol transitions will occur;
because the phase jumps can be ±90°, we can’t use the approach described for
the 180° phase jumps produced by BPSK modulation.
This
problem can be mitigated by using one of two QPSK variants. Offset QPSK, which
involves adding a delay to one of two digital data streams used in the
modulation process, reduces the maximum phase jump to 90°. Another option is
π/4-QPSK, which reduces the maximum phase jump to 135°. Offset QPSK is thus
superior with respect to reducing phase discontinuities, but π/4-QPSK is
advantageous because it is compatible with differential encoding (discussed in
the next subsection).
Another
way to deal with symbol-to-symbol discontinuities is to implement additional
signal processing that creates smoother transitions between symbols. This
approach is incorporated into a modulation scheme called minimum shift keying
(MSK), and there is also an improvement on MSK known as Gaussian MSK.
Differential
Encoding
Another
difficulty is that demodulation with PSK waveforms is more difficult than with
FSK waveforms. Frequency is “absolute” in the sense that frequency changes can
always be interpreted by analyzing the signal variations with respect to time.
Phase, however, is relative in the sense that it has no universal reference—the
transmitter generates the phase variations with reference to a point in time,
and the receiver might interpret the phase variations with reference to a
separate point in time.
The
practical manifestation of this is the following: If there are differences
between the phase (or frequency) of the oscillators used for modulation and
demodulation, PSK becomes unreliable. And we have to assume that there will be
phase differences (unless the receiver incorporates carrier-recovery
circuitry).
Differential
QPSK (DQPSK) is a variant that is compatible with noncoherent receivers (i.e.,
receivers that don’t synchronize the demodulation oscillator with the
modulation oscillator). Differential QPSK encodes data by producing a certain
phase shift relative to the preceding symbol. By using the phase of the
preceding symbol in this way, the demodulation circuitry analyzes the phase of
a symbol using a reference that is common to the receiver and the transmitter.
Summary
•Binary
phase shift keying is a straightforward modulation scheme that can transfer one
bit per symbol.
•Quadrature
phase shift keying is more complex but doubles the data rate (or achieves the
same data rate with half the bandwidth).
•Offset
QPSK, π/4-QPSK, and minimum shift keying are modulation schemes that mitigate
the effects of high-slope symbol-to-symbol voltage changes.
•Differential
QPSK uses the phase difference between adjacent symbols to avoid problems
associated with a lack of phase synchronization between the transmitter and
receiver.
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Source:
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